To produce recombinant proteins cost efficiently at industrial scale it is desirable to have them secreted. This allows the proteins to be accumulated extracellularly in high amounts that be easily recovered without contamination by exogenous cellular or DNA material.
There is a constant interest, driven by industrial need, to find more effective vehicles for protein secretion.
In both prokaryotes and eukaryotes, signal peptides govern the entry of virtually all proteins to the secretory pathway and thus may affect protein production levels.
A signal peptide is a short peptide chain—typically 3-60 amino acids long—that directs the post-translational transport of a protein. Signal peptides may also be called targeting signals, signal sequences, transit peptides, or localization signals.
The amino acid sequences of signal peptides direct proteins (which are synthesized in the cytosol) to certain organelles such as the nucleus, mitochondrial matrix, endoplasmic reticulum, chloroplast, apoplast and peroxisome. Some signal peptides are cleaved from the protein by signal peptidase after the proteins are transported.
Though variable in primary sequences, signal peptides are quite universal: they can function across the species and even in different organisms.
This opens up the possibility of constructing a universal expression vector that can provide a high production level of recombinant proteins secreted into culture media in a broad range of hosts.
So far, the most commonly used signal sequence in several yeast species, including Saccharomyces cerevisiae, Hansenula polymorpha, Pichia pastoris is one derived from the alpha factor peptide of S. cerevisiae. 
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